A major problem associated with the use of highly unsaturated rubbers (such as natural rubber, polybutadiene, styrene-butadiene rubber, nitrile rubber, polyisoprene and the like) is the surface degradation, typically exemplified by cracking, caused by ozone. In order to overcome this problem of ozone attack, several approaches have been undertaken to increase the ozone resistance of such highly unsaturated rubbers.
One such approach has been to blend the highly unsaturated rubber with an ozone resistant rubber, such as high molecular weight EPDM. Thus, Kirk-Othmer, Encyclopedia of Chemical Technology, 3d Ed. Vol 8, pg. 499 (1980), indicates that " . . . an important application for EPDM is in blends with another rubber. Ozone resistance is thus provided without significant participation in co-cure, with the host rubber comprising the major portion of the blend." This use of EPDM to increase the ozone resistance of unsaturated rubbers is the subject of several publications, many directed to co-curing mechanisms, such as U.S. Pat. No. 3,706,819 (Usamoto et al), U.S. Pat. No. 3,678,135 (Mastromatteo et al) and the like.
A second approach taken in the past to improve the ozone resistance of highly unsaturated rubbers has been to add chemicals to combat the attack of ozone. Among the classes of compounds which have been employed with some success are quinolines. Thus, Kirk-Othmer, supra, Vol. 20 at page 407, states that " . . . styrenebutadiene rubber, polybutadiene, nitrile, polyisoprene and natural rubber are quite susceptible to ozone attack . . . and 1,2-dihydro-2,2,4-trimethyl-6-ethoxyquinoline (Santoflex A. W. Monsanto) give[s] adequate ozone protection". Additional quinoline antidegradants are described in the "Index of Commercial Antioxidants & Antiozonants," Goodyear Chemicals (3d Ed., 1982).
The combination of these approaches has also been attempted in the past. Thus, U.S. Pat. No. 3,356,764 to Gentile discloses an ozone-resistant composition comprised of (1) conjugated diene polymer rubber, (2) ethylene-propylene copolymer rubber (preferably EPDM), and (3) poly(trimethyl dihydroquinolene) or 6-ethoxy-1,2-dihydro-2,2,4-trimethyl quinoline. While such combination of approaches will improve the ozone resistance of highly unsaturated rubbers to some degree (compare Comparative Experiments A and B below), it would nevertheless be desirable to further increase the ozone resistance of such rubber compositions, and/or to be able to increase the proportion of unsaturated rubber which comprise such mixture (and thereby increase the tensile properties of the composition) while maintaining good ozone resistance.
It has now been unexpectedly found that when the EPDM component of compositions similar to those described in Gentile (i.e., compositions comprised of a highly unsaturated rubber, EPDM and a quinoline antidegradant) is composed of two components--(a) a high molecular weight component, similar to the material employed in Gentile; and (b) a low molecular weight "liquid" EPDM component (a substance unknown at the time of the Gentile patent)--the ozone resistance of such composition is significantly increased at similar EPDM levels. Moreover, it has been discovered that the use of such high molecular weight and low molecular weight EPDM components will unexpectedly reduce the viscosity and therefore increase the processability of the uncured composition.